The earliest EEG signatures of object recognition in a cued-target task are postsensory.

Recent experiments have demonstrated early target minus nontarget differences in the human event-related potential (ERP) during visual object recognition tasks. It is unclear whether these differences reflect high-level visual processes, effectively indexing the speed of object recognition, or whether they arise from postsensory decision processes, leaving the actual time of object recognition uncertain. Here we report three sets of ERP experiments designed to determine what processes underlie the target minus nontarget difference signals seen in visual cued-target paradigms. We demonstrate that the same difference signals are present when the target match is made to word stimuli as well as to object stimuli, suggesting that the disparate mechanisms involved in letter string and object processing are not directly responsible for the signals. We also find that the amplitude of these signals can be reduced by increasing trial difficulty in three different ways: image difficulty, level of semantic categorization, and overall task demands. In many respects, the difference signal is similar to the postrecognition P300. Together, these results suggest that the target minus nontarget difference does not reflect object recognition per se, but rather postsensory decision processes.